Related (yar-har!) to chin development in humans can be influences, in both directions, to/from diet and vocalizations. Dunno how it fits with elephants or manatees, but we definitely got top agriculture, access to flour, and heavy reliance on language. Had something more, but can't find the bookmark atm.
"When laypersons assessed, a significant correlation was found between facial attractiveness and the chin (r = 0.671), eyes (r = 0.669), lips (r = 0.585), hair (r = 0.527), teeth (r = 0.338), and nose (r = 0.247); the chin was responsible for 45.1% of the variation in facial attractiveness, the eyes for 14.3%, and the lips for 0.8%. " The argument put forth by Wrangham is the mandible became a truly pussy organ as soon as we learned to cook. The argument put forth by Harari is that group formation is what led to dominance of Homo Sapiens over Homo Neanderthalensis. Chins visually emphasize the smile; as muscle groups gave over to communication from mastication, chins were inevitable. Even before you consider that they're covered with a secondary sex characteristic in males. the ONE place males humans grow hair that females don't includes the chin.
The author of the paper about chin's explanation, argue in this interview, than speach and chewing, is not enough https://italy.timesofnews.com/health-care/why-do-humans-have-chins-a-scientist-explains-the-enduring-puzzle I was wrong about elephant (and probably manatee) they dont have one : https://www.npwrc.org/why-do-humans-have-chins-a-scientist-explains-the-enduring-puzzle.html